1. Field of the Invention
The present invention relates to a ferrite substrate for thin-film inductors, to a thin-film common mode filter using the substrate, to a thin-film common mode filter array using the substrate, and to a manufacturing method of the substrate.
2. Description of the Related Art
Common mode filter is a device for suppressing common mode currents that cause electromagnetic interference in parallel transmission lines. The common mode filter has magnetically coupled inductors to remove in-phase noise component.
Thin-film common mode filter miniaturized and highly integrated by forming bilayered thin-film coils between ferrite substrates and by constructing in chip form, and thin film common mode filter array on which a plurality of the filters are mounted, are described in for example, Japanese Patent Publications Nos. 08-203737A and 11-054326A.
Generally, such a ferrite substrate is produced by hot forming press method where a hot-pressed block is cut out into substrates with a desired shape and the substrates are then lapped and formed, or by sheet manufacturing method where sheeted ferrites are stacked and pressed with heat and the stacked ferrite is then lapped and formed into a desired shape.
In the thin-film common mode filter, coils are disposed closely to each other in order to satisfy it's characteristic request and high voltage is applied to these coils. Thus, such filter is required to have high withstand voltage and high reliability in electrical isolation. Also, required is that terminals of the filter should be electrically isolated with each other and formed finely without causing electrical isolation failure between coils. Furthermore, the filter should have miniaturized coils and ferrite substrates with a permeability of about 100-400 in order to be operable at a high frequency (several GHz) band.
Conventional ferrite substrate for the thin-film common mode filter, however, has a porous crystalline structure with voids and such on its surface, which causes low insulation resistance on its surface and large surface-degradation. The ferrite substrate, therefore, has too poor mechanical strength to undergo thin-film process, and moreover, it has been difficult to form precisely the terminals on the substrate surface.